1. Field of the Invention
The invention in general relates to the field of medical pneumatic tourniquets, and more particularly to a tourniquet which is automatically controlled to maintain the desired tourniquet pressure.
2. Description of the Prior Art
Tourniquets are used in medicine to stop the flow of blood into a patient's limb. A typical use is to provide a "bloodless field" in which surgical procedures may be performed on a limb, without the field being obscured by blood, and without loss of blood to the patient. Tourniquets are also used in emergency procedures to prevent loss of blood when a limb is damaged or in any other instance where it is desired to prevent blood flow to a limb.
Tourniquets conventionally consist of an inflatable cuff which is wrappd around the patient's limb, and a source of compressed gas for inflating the cuff. In order to successfully stop the flow of blood the pressure in the cuff must be above the highest level of the patient's blood pressure. However, it should not be so high that it damages the tissue of the limb. Thus in conventional tourniquets a pressure gauge for measuring the cuff pressure, and a pressure regulating mechanism is provided.
Up until very recently, conventional pneumatic tourniquets have been manually controlled mechanical devices. That is, a mechanical mechanism visually indicated the pressure and the pressure was adjusted by an attendant using a mechanical valve.
Recently, an automatic tourniquet for medical use has been created by Dr. James A. McEwen. This tourniquet is described in U.S. patent application Ser. No. 193,145 filed Oct. 2, 1980. An improved automatic tourniquet is described in U.S. patent application Ser. No. 337,152 filed Jan. 5, 1982, also by Dr. James McEwen. In these automatic tourniquets, the pressure in the cuff is continuously sensed and adjusted and displayed by a microprocessor. In addition, the microprocessor activates alarms if a dangerous pressure condition is sensed. It has been found that such automatic tourniquet systems not only are much more convenient in the operating room setting, but also significantly reduce the incidence of limb paralysis, nerve damage and other injuries attributable to tourniquets.
The preferred pressure measuring unit for hospital tourniquet systems is the millimeter of mercury (mmHg). The maximum tourniquet pressures are in the range 400 to 500 mmHg. Thus a preferred scale for measurement of pressure is from 0 to about 500 mmHg. In order to read such a scale in single units of mmHg, a digital system having processing capabilities for 9 bits of information is preferable, since an 8 bit system is capable of expressing only 2.sup.8 =256 unique integers, while a 9 bit system is capable of handling 2.sup.9 =512. At the same time, it has developed that the microprocessor industry has standardized components available in an 8 bit mode.